It is generally known to use hydraulic systems, for example for configuring actuators for multi-stepped transmissions in motor vehicles. In such semi-automatic transmissions the hydraulic actuator displaces a gearshift rod along a first actuating direction and, further, in a second actuating direction perpendicular to the first actuating direction for engaging four or five or six forward gears plus one reverse gear.
Hydraulic actuators of the afore-mentioned kind are generally equipped with piston-and-cylinder units having chambers for displacing a piston or a diaphragm which, in turn, is used to displace the gearshift rod or other comparable components. A piston-and-cylinder unit may be configured as a double-acting unit in which the piston adjoins two hydraulic pressure chambers. The pressure in these chambers may be set such that the force exerted on the piston from both sides results in a predetermined axial movement of the piston. In many cases pressure is applied from one side of the piston only in order to displace the piston accordingly. However, it is also possible to establish a differential pressure or to configure the surfaces of the piston such that the resultant force exerted on the piston effects the desired axial movement.
In other cases, the piston-and-cylinder unit may be configured as a single-acting unit in which hydraulic pressure is only applied to one side of the piston whereas the opposite side of the piston is biased by a spring.
If hydraulic pressure is applied to a cylinder chamber in piston-and-cylinder units, the piston will be displaced accordingly and the piston displacement may be transmitted to a corresponding displacement of, for example, a piston rod in one of the two above-mentioned directions. It is, therefore, possible to thus configure a system in which a conventional manually operated multi-stepped transmission, after having been provided with such hydraulic actuator, may be converted into a fully-automatic or a semi-automatic transmission.
In hydraulic systems of the kind of interest it is a general problem that due to leaks in the system gas, in particular air, may penetrate into the hydraulic system. Considering that air is compressed in volume as soon as pressure is applied thereon, in contrast to liquids which are non-compressible, one may easily appreciate that the physical properties as well as the performance characteristics of a hydraulic actuator may be severely affected when air or gas find its way into the system.
It is well known in the art to degas hydraulic systems by providing degassing valves. Such degassing valves allow the air or the gas being dispersed in the hydraulic liquid to escape the system because when the degassing valves are opened the hermetic seal between the interior of the hydraulic system and ambient temperature is briefly deactivated. Prior art degassing valves are generally configured as screw-type valves. For degassing a hydraulic system, the operator will turn the screw-type degassing valve so that the air which is trapped within the system will escape under the action of the hydraulic fluid pressure. When the air has escaped, foam-type hydraulic fluid will exit from the degassing valve. As soon as pure hydraulic fluid becomes visible, the operator will again close the degassing valve and the actuator or the hydraulic system may be considered as de-gassed so that the valve may be closed again.
Degassing valves of conventional design are normally not easily accessible. Generally, for degassing a hydraulic sytsem, two persons are required. One of the persons will operate the hydraulic system for establishing the required hydraulic fluid pressure, whereas the other person will operate the degassing valve.
It is, therefore, an object of the present invention to improve a hydraulic system of the above-mentioned kind so that degassing of the system is facilitated.
More specifically, it is an object underlying the invention to create a method for degassing such hydraulic systems which is simple to execute and may also be used by unskilled persons.
Still more specifically, it is an object underlying the present invention to improve a motor vehicle transmission of the fully-automatic or semi-automatic type based on conventional manually operated multi-stepped transmissions and having a hydraulic actuator so as to obviate any problems connected with the presence of air or gases trapped within the hydraulic actuator of such transmission systems.